This application claims Paris Convention priority of European patent application number 00123824.5 filed Nov. 2, 2000, the complete disclosure of which is hereby incorporated by reference.
The invention concerns a method for determining measurement tolerances of an optical spectrum analyzer by means of at least one reference line of a known wavelength and known spectral bandwidth, and an optical spectrum analyzer which is suitable for carrying out this method.
It is known that the wavelength display of optical spectrum analyzers (OSA) is re-calibrated with a reference element during operation since the displayed wavelength changes due to aging, temperature, shock and other influences.
EP 0 758 075 discloses calibration of an optical spectrometer by means of one or more reference lines with respect to the absolute wavelength. These reference lines are the minimum in a reference spectrum and very stable. Gas absorption lines are used as reference lines in this case.
U.S. Pat. No. 5,838,437 discloses another possibility of absolute wavelength calibration. Therein, the absolute wavelength calibration is carried out through temperature-stable Fabry-Perot filters and fiber gratings.
These known methods calibrate the OSA only with respect to the absolute wavelength. It is, however, possible that the spectral bandwidth of the OSA changes due to mechanical tolerances, temperature changes, shock, aging or other influences. These changes are not detected by the conventional calibration methods.
A spectral bandwidth which is not exactly known causes measurement errors when measuring the OSNR (optical signal to noise ratio) in optical systems. OSNR is usually measured between a modulated laser and noise. Identical measurements require identical optical bandwidths. Since different OSAs usually have different spectral bandwidths, the spectral bandwidth is usually standardized to 0.1 nm (wherein the noise performance must be converted between actual bandwidth and 0.1 nm). Therein, the spectral bandwidth is the wavelength difference of the points, at which the measured performance has dropped by 3 dB from the maximum value to a higher or lower wavelength. If the actual spectral bandwidth of the OSA changes during operation, the determined OSNR is wrong.
It is therefore the underlying object of the present invention to provide a method for spectral bandwidth calibration of optical spectrum analyzers during operation and provide a corresponding optical spectrum analyzer.
This object is achieved in accordance with the invention in that, from the known spectral bandwidth of the at least one reference line and from the spectral bandwidth of the at least one reference line measured with the optical spectrum analyzer, the respective spectral bandwidth of the optical spectrum analyzer is determined at the wavelength of the at least one reference line. Preferably, several spectral bandwidths of the optical spectrum analyzer are detected by means of several reference lines of a known wavelength and known wavelength bandwidth.
The measured spectral bandwidth xcex94xcexMi of a reference line i (i=1,2, . . . ) is calculated in approximation from the spectral bandwidth xcex94xcexRi of this reference line and from the spectral bandwidth xcex94xcexOSi of the optical spectrum analyzer:
xcex94xcexMi={square root over (xcex94xcexRi2+xcex94xcexOSi2)}
(i=1, 2, . . . ), and therewith the spectral bandwidth xcex94xcexOSi of the optical spectrum analyzer at the reference line to:
xcex94xcexOSi={square root over (xcex94xcexMi2xe2x88x92xcex94xcexRi2)}
(i=1, 2, . . . ).
The advantage achieved with the invention consists in that spectral bandwidth changes during operation are determined and taken into consideration by suitable recalibration during operation. In this fashion, erroneous measurements of optical spectra can be avoided.
The inventive method of bandwidth calibration can be applied to all kinds of optical spectrum analyzers, such as e.g.
gratings with individual detector, wherein the spectrum is recorded through changing one mechanical variable, e.g. rotation of the grating,
gratings with detector array wherein each element of the array records one point of the spectrum; or
Fabry-Perot resonator whose length can be tuned thereby scanning the spectrum.
The reference lines in the reference spectrum can either be the maximum or minimum.
The spectral bandwidths determined at the reference lines can be stored either as calibration data of the optical spectrum analyzer and therefore be taken into consideration immediately with each measurement. Alternatively, a measurement spectrum measured with the optical spectrum analyzer can be corrected later by means of the spectral bandwidths determined at the reference lines.
If the spectral bandwidths xcex94xcexRi of the reference lines i (i=1, 2, . . . , n) are considerably smaller than their measured spectral bandwidths xcex94xcexMi, i.e. if xcex94xcexRi less than xcex94xcexMi, the respective spectral bandwidth xcex94xcexOSi of the optical spectrum analyzer can be set approximately identical to the measured spectral bandwidth xcex94xcexMi of the ith reference line. Otherwise, e.g. if xcex94xcexRixe2x89xa7xcex94xcexMi, the spectral bandwidth of the optical spectrum analyzer can be detected e.g. through deconvolution.
The spectral bandwidths at other wavelengths can be determined through interpolation or extrapolation of the determined spectral bandwidths. The measurement of the reference spectrum and the measurement of the measurement spectrum can be carried out simultaneously or one after the other.
The inventive optical spectrum analyzer for carrying out the above-described method comprises a reference light source, a first storage medium for storing the spectral bandwidths of known reference lines of the reference light source, a spectral bandwidth determination means for determination of the spectral bandwidths of the reference lines measured by the spectrometer and a spectral bandwidth determination means for determining the spectral bandwidths of the optical spectrum analyzer from the known spectral bandwidths of the reference lines and from the spectral bandwidths of the reference lines measured by the optical spectrum analyzer at the respective wavelengths of the reference lines.
A preferred embodiment of the inventive optical spectrum analyzer provides a second storage means which stores the determined spectral bandwidths of the optical spectrum analyzer at the respective wavelengths of the reference lines. Intermediate values and boundary values can be gained through interpolation and extrapolation of the determined spectral bandwidths.
The spectral bandwidths in the second storage medium are preferably initially stored through manufacturer calibration which are then recalculated and correspondingly changed on the basis of the newly gained spectral bandwidths.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.